The present invention relates to a submarine, optical-fiber cable.
FIG. 8 is a transverse cross section of a conventional, submarine, optical-fiber cable having 48 optical fibers. This cable has been actually used in submarine, optical communication in Japan.
Referring to FIG. 8, an assembly unit 82 has a center wire 80 in its center, optical fiber units 81 surrounding the center wire 80, and a resin compound filling any otherwise empty space among them in the assembly unit 82. The optical fiber units 81 include a plurality of optical fibers. Strength members 84a and 84b for resisting tension are disposed around the assembly unit 82. A pressure-resisting tube 85, which may be a copper outer pipe, surrounds the strength members 84a and 84b. A compound 83 for preventing the ingress of water fills the spaces which are within the pressure-resisting tube 85 and outside of the assembly unit 82, and which include spaces between the strength members 84a and 84b. A polyethylene sheath 86 covers the outer surface of the pressure-resisting tube 85, and another polyethylene sheath 87 covers the polyethylene sheath 86.
The submarine, optical-fiber cable is subjected to considerable hydrostatic pressures. In order to protect the optical fibers from compressive stresses in the radial direction, the optical-fiber cable needs a structure to resist the stresses. A pressure-resisting tube 85 made of copper or aluminum is used in the cable of FIG. 8 for this purpose.
The humidity around optical fibers in the cable may increase underwater because water permeates the polyethylene sheaths 86 and 87. Optical fibers are more prone to break under a wet environment than under a dry environment because a minute crack on the surface of optical fibers is more susceptible to grow. To prevent the potential breakage of the optical fibers, a pressure-resisting tube 85 has a hermetic structure without pin holes produced by a welding process.
However, in the process for manufacturing the cable, water may have been adsorbed on surfaces of strength members 84a and 84b and the inner surface of the outer pipe 85. During a step of coating the polyethylene sheath 86, heat is applied and some of the adsorbed water is released into a vapor phase due to the heat. Thus the vaporized water may reach a saturated vapor pressure to form a small amount of dew inside the pressure-resisting tube 85.
Ingress of sea water into the cable due to breakage of pressure-resisting tube 85 may cause corrosion to strength members 84a and 84b to result in the generation of hydrogen, and hydrogen increases optical loss of fibers. Thus it is important to prevent the ingress of water in the cable. Ingress of sea water into the cable may be caused by wear of the cable. Alternatively, such ingress may occur when the cable is damaged or cut off by an anchor thrown from a ship.
Therefore, to prevent water from contacting fiber unit 81 in the conventional cable, a resin that hardens upon the irradiation of ultraviolet rays fills up the otherwise empty spaces around the center wire 80 in the assembly unit 82 in FIG. 8.
Moreover, to prevent water from spreading to the spaces inside the pressure-resisting tube 85 and outside the assembly unit 82, a compound 83 for preventing the ingress of water, such as a polyurethane resin, is provided in the spaces, leaving some intervals between dams made of the compound. To pack a compound 83 for preventing the ingress of water to form a dam requires skills of the highest degree in the process for making the cable. To limit the spread of water entered into the cable within 1 kilometer in a month, all the sum of the cross section of the water passes in the cable should be smaller than the cross section of one water pass having a diameter of 10 .mu.m.
The assembly unit 82 has a function to prevent optical fibers from slipping during laying and recovering a cable. When the cable is laid to or raised from underwater at the depth of a few thousand meters below the sea level, the cable is subjected to a considerable fluctuating tension, which is proportional to the weight of the cable. The tension may reach to 10,000 kgf. Thus the optical fibers in the cable are subject to an elongation strain larger than 0.5%. The assembly unit 82 makes the behavior of optical fibers agreeable to that of the cable.
To sum up, the conventional submarine, optical-fiber cable is characterized in:
(1) that the cable includes a pressure-resisting tube, which has a hermetic structure; PA1 (2) that the cable includes a assembly unit; and PA1 (3) that the cable includes dams made of a compound for preventing water in the spaces inside the pressure-resisting tube 85 and outside the assembly unit 82. PA1 r.sub.1 is the distance between the center axis of the cable and the center axis of the optical fiber; and PA1 r.sub.2 is the radius of curvature of the cable.
The optical fiber unit 81 usually has six to twelve optical fibers, and the maximum number of the optical fibers in the cable is limited to forty eight.
A buckling pressure, P, of a pressure-resisting tube having an average radius of r is given by the following equation (1): EQU P={E/4(1-.nu..sup.2)}(t/r).sup.3 ( 1)
wherein the thickness, the Young's modulus, and the Poisson's ratio of the tube are t, E, and .nu., respectively.
One embodiment of the cable in FIG. 8 has a pressure-resisting tube or an outer pipe 85 made of copper, having an outer diameter of about 11.4 mm. Thus, the buckling pressure of the tube alone is calculated according to the equation (1) to be about 200 atms. Since the buckling pressure is proportional to the inverse of the cube of the average radius according to the equation (1), the increase in the number of optical fibers leads to a larger diameter of the assembly unit 82, which further increases the radius of the pressure-resisting tube so that the buckling pressure of the tube decreases. Therefore, the number of optical fibers accommodated in the conventional cable is inherently limited.
Bending a cable causes a bending strain .epsilon. in the optical fibers in the cable. The bending strain .epsilon. in an optical fiber is given by the following equation (2): EQU .epsilon.=r.sub.1 /r.sub.2 ( 2)
wherein
The cable of FIG. 8 having 48 optical fibers has r.sub.1 of 2.5 mm. During laying of the cable, the cable is subject to a deflection with the radius of curvature of about 0.5 meters at the sheave in the ship. Thus according to equation (2), the bending strain of the optical fiber on this occasion reaches to 0.5%.
The further increase in the number of optical fibers in the cable leads to the increase in the bending strain of the optical fibers, raising the probability that optical fibers will be cut off. Moreover, it is technically difficult to guarantee reliability of the cable having the structure with increased number of optical fibers over a long period of time. Therefore, it is difficult to increase the number of the optical fibers in the cable of FIG. 8 to more than forty eight.
The increase in the number of optical fibers in the cable of FIG. 8 prolongs the time required to splice optical fibers. To connect between optical fibers, fine, fragile optical fibers in a fiber unit have to be separated from the fiber unit. It takes time to complete this separation, and the time required for the separation increases as the number of optical fibers increases. Moreover, each of the optical fibers in the cable of FIG. 8 is discrete so that each fiber has to be separated from the unit and then spliced to another fiber one by one. As a result it takes about eight hours to splice optical fibers in the cable of FIG. 8. Furthermore, it is not easy to recognize each fiber because the fiber is so thin. The repair of the cable has to be done within 24 hours at sea partly because the weather may not be stable. Therefore, it is difficult to increase the number of the optical fibers in the cable of FIG. 8 to more than forty eight from this point of view also.
For all the reasons mentioned above the maximum number of optical fibers in the cable of FIG. 8 is limited to forty eight.
On the other hand, FIG. 9 is an example of a transverse cross section of a conventional, land, optical-fiber cable having 100 optical fibers. This cable has a slotted rod 93 surrounding a center wire 94. Each of the slots on the rod 93 has five optical ribbons 91 stacked together, and each of the ribbons 91 contains four optical fibers. A tape 92 for absorbing water is disposed around the slotted rod 93, and a polyethylene sheath 95 surrounds the absorbent tape 92. A pair of fibers 96 are disposed in one of the slots. The cable shown in FIG. 9 does not contain a hermetic metal tube for resisting pressure so that the cable is not suitable for its application in a submarine environment.